Fonseka's side said Sunday there was evidence of a coup plot by Rajapakse's ruling party to try to hold on to power but the government denied any intention to subvert the electoral process.
Rajapakse called Tuesday's vote after only four years of his six-year mandate to try to harness public acclaim for victory over the Tiger rebels, who controlled one-third of Sri Lanka just 10 years ago.
The conflict between government troops and the Tigers cost between 80,000 and 100,000 lives, according to UN estimates. At least four people have been killed and hundreds wounded in the run-up to Tuesday's vote, being held after the separatist Tamil Tiger rebels were defeated by government troops in May, ending almost four decades of conflict.
Incumbent President Mahinda Rajapakse, who ordered the bloody military offensive that crushed the Tigers, is facing the strongest challenge from Sarath Fonseka, the army chief who led the troops in battle.
Rajapakse urged the authorities on Monday to conduct a